Ultra-clean atmospheric pressure chemical vapor deposition (APCVD) reactors are used conventionally in the fabrication of semiconductor devices. In the reactor, reactant gases are brought together to carry on a reaction over a silicon wafer. Using this technique, it is possible to deposit layers of polysilicon, silicon dioxide, or other compounds on the wafer. A typical horizontal tube-type APCVD consists of a horizontal quartz tube having reactant gases metered into one end of the tube and unused or bi-product gases exhausted out the other end.
With ultra-clean APCVD reactors, it is necessary to prevent oxygen, water vapor, and other contaminants from the ambient atmosphere from entering the reactor. In some applications, the oxygen and water vapor levels must be kept below 10 parts per billion. A high purity environment such as that in an ultra-clean APCVD reactor is known as a load-locked environment.
Maintaining the high purity level in an ultra-clean APCVD becomes problematic when the reactor is shutdown as a result of a power failure or a fault in any of the systems associated with the reactor. Two techniques are currently used to maintain the purity of the reactor during a shutdown.
The first of these techniques is known as the closed-system technique. During a shutdown, the closed-system technique uses valving to shut off the gas flow to and from the reactor automatically. In this way, contaminants are prevented from entering the reactor. A problem arises, however, as the reactor cools from its operational temperature of as high as 1200.degree. C. to room temperature. As a result of the temperature drop, there is a corresponding drop in pressure within the reactor from the operational pressure of approximately atmospheric pressure. The pressure decrease inside the reactor can result in an implosion pressure of as high as 12 psi exerted by the atmosphere on the reactor vessel. Such a pressure can easily rupture a typical quartz reactor tube with rectangular cross-section. Laboratory reactor tubes have been known to explode with outward overpressure differentials of 3 to 7 psi. Aside from the material loss, reactor tube rupture of APCVD's is particularly dangerous because of the presence of toxic and/or flammable gases typically used in APCVD reactors.
The second technique used to prevent influx of ambient contaminants is known as the open-system technique. Here, only the inlet valve to the reactor from the toxic and/or flammable sources is discontinued automatically. The reactor outlet remains open and an inert gas purge such as nitrogen is started into the reactor that purges the reactor contents into the exhaust duct work. Although the open-system technique avoids the problems of reactor implosion associated with the closed-system technique, it has two other distinct problems associated with it.
First, if the system faults and the reactor shuts down due to an exhaust failure, the contents of the reactor, including toxic and/or flammable gases, are discharged into the non-functioning exhaust. In this event, it is possible that the toxic gases discharged from the reactor could enter laboratory or work space and become a significant safety hazard.
Second, the open-system technique requires a constant inert gas flow. If the inert purge is ever discontinued (for example, if the purge or the purge gas source needs to be shut down for maintenance), particles from the ambient environment can enter and contaminate the reactor. This results in significant down-time in order to dry the system before normal operation can be resumed.